1) It is very hard at this point to guess what the problem might be concerning your false trigger problem. I cannot envision any other type of things from my past experence. Is there some large amount of noise on Vcc or GND lines at the processor that is maybe making the false trigger happen?

2) When the false trigger occurs is it possible for you to look at the capture register values and determine if the false captures are occurring at consistent counts from the previous "known good" captures? Or are the occurrances happening at random intervals?

3) One method to do further debug.....make a test version of your software that interrupts on each capture from this pin. Then in the ISR put in two lines of extra code to toggle another spare output pin on and off. Use this pulse as a scope trigger on a DSO (digital storage oscilloscope) on an end frame trigger or center frame trigger. Using this as a reference you can probe other parts of the circuit to look for glitches on say GND, that may be leading to the false trigger. You may also be able to learn some additional character about the false trigger by seeing on a scope the positioning of this output pulse relative to the input pulse pins.

4) Is there any possibility that you might by chance have some type of strange software problem? Sometimes it can be helpful to reduce the mis-behaving part down to a bare minimum code and test with that only.

5) The emitter follower configuration of the opto coupler like I showed in my sketch can lead to wider bandwidth through the opto coupler. (You may want to fetch some of the 2701 style opto parts from Digikey and try them out. I believe they are the same pinout as the 2501. The 2701 is a slightly less sensitive part, i.e. the CTR is lower).

6) I made the little sketch as follows: I opened the NEC data sheet for the PS2701 coupler in Acrobat. There was a figure in the data sheet showing the test circuit. I zoomed the PDF file to the desired level. I then use a cool utility I have called Kleptomania that allows for screen capture in an interactive and selective manner. (See demo at www.structurise.com/kleptomania/ ). This captures a bitmap to the clipboard. I then opened the graphic in MS Paint program and pasted the clip board in to the paint program. Using paint I modified the capture a bit and changed some component values and made the picture a bit smaller by squeezing it together. Finally I used the paint program to save the graphic as a .GIF file.
That file was then uploaded to my 8052.com page and linked into the forum post.

7) For a generic graphics programs I use any of Corel Draw, Autocad, Visio, MS Paint or Adobe Photoshop as to what is most applicable to what I am trying to create.

8) Work in laser stuff is VERY fun.....

Michael Karas